Course Name: Database Management Systems

Lecture 3 Topics to be covered

Data Models

DBMS Languages

2

Data Models

Data Model: A set of concepts to describe the structure of a database, and certain constraints that the database should obey.

Data Model Operations: Operations for specifying database retrievals and updates by referring to the concepts of the data model. Operations on the data model may include basic operations and user-defined operations.

Categories of data models

Conceptual (high-level, semantic) data models: Provide concepts that are close to the way many users perceive data. (Also called entity-based or object-based data models.)

Physical (low-level, internal) data models: Provide concepts that describe details of how data is stored in the computer.

Implementation (representational) data models: Provide concepts that fall between the above two, balancing user views with some computer storage details.

History of Data Models

Relational Model: proposed in 1970 by E.F. Codd (IBM), first commercial system in 1981-82. Now in several commercial products (DB2, ORACLE, SQL Server, SYBASE, INFORMIX).

Network Model: the first one to be implemented by Honeywell in 1964-65 (IDS System). Adopted heavily due to the support by CODASYL (CODASYL - DBTG report of 1971). Later implemented in a large variety of systems - IDMS (Cullinet - now CA), DMS 1100 (Unisys), IMAGE (H.P.), VAX -DBMS (Digital Equipment Corp.).

Hierarchical Data Model: implemented in a joint effort by IBM and North American Rockwell around 1965. Resulted in the IMS family of systems. The most popular model. Other system based on this model: System 2k (SAS inc.)

History of Data Models

Object-oriented Data Model(s): several models have been proposed for implementing in a database system. One set comprises models of persistent O-O Programming Languages such as C++ (e.g., in OBJECTSTORE or VERSANT), and Smalltalk (e.g., in GEMSTONE). Additionally, systems like O2, ORION (at MCC - then ITASCA), IRIS (at H.P.- used in Open OODB).

Object-Relational Models: Most Recent Trend. Started with Informix Universal Server. Exemplified in the latest versions of Oracle-10i, DB2, and SQL Server etc. systems.

Hierarchical Model

• ADVANTAGES:

• Hierarchical Model is simple to construct and operate on

• Corresponds to a number of natural hierarchically organized domains - e.g., assemblies in manufacturing, personnel organization in companies

• Language is simple; uses constructs like GET, GET UNIQUE, GET NEXT, GET NEXT WITHIN PARENT etc.

• DISADVANTAGES:

• Navigational and procedural nature of processing

• Database is visualized as a linear arrangement of records

• Little scope for "query optimization"

Network Model

• ADVANTAGES:

• Network Model is able to model complex relationships and represents semantics of add/delete on the relationships.

• Can handle most situations for modeling using record types and relationship types.

• Language is navigational; uses constructs like FIND, FIND member, FIND owner, FIND NEXT within set, GET etc. Programmers can do optimal navigation through the database.

• DISADVANTAGES:

• Navigational and procedural nature of processing

• Database contains a complex array of pointers that thread through a set of records. Little scope for automated "query optimization”

Schemas versus Instances

• Database Schema: The description of a database. Includes descriptions of the database structure and the constraints that should hold on the database.

• Schema Diagram: A diagrammatic display of (some aspects of) a database schema.

• Schema Construct: A component of the schema or an object within the schema, e.g., STUDENT, COURSE.

• Database Instance: The actual data stored in a database at a particular moment in time. Also called database state (or occurrence).

Database Schema Vs. Database State

• Database State: Refers to the content of a database at a moment in time.

• Initial Database State: Refers to the database when it is loaded

• Valid State: A state that satisfies the structure and constraints of the database.

• Distinction

• The database schema changes very infrequently. The database state changes every time the database is updated.

• Schema is also called intension, whereas state is called extension.

DBMS Languages

Data Definition Language (DDL): Used by the DBA and database designers to specify the conceptual schema of a database. In many DBMSs, the DDL is also used to define internal and external schemas (views). In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas.

DBMS Languages

• Data Manipulation Language (DML): Used to specify database retrievals and updates.

• DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language), such as COBOL, C or an Assembly Language.

• Alternatively, stand-alone DML commands can be applied directly (query language).

DBMS Languages

• High Level or Non-procedural Languages: e.g., SQL, are set-oriented and specify what data to retrieve than how to retrieve. Also called declarative languages.

• Low Level or Procedural Languages: record-at-a-time; they specify how to retrieve data and include constructs such as looping.